This invention relates to a method of preparing a group of aromatic polyamides which are represented by the general formula (I) and which can be converted into corresponding polybenzoxazoles by appropriate heating: ##STR1## wherein X is a divalent organic group, m is 0 or 1, R is a divalent organic group, and n is an integer from 1 to 200. For example, the organic group X is --C(CH.sub.3).sub.2 -- or --C(CF.sub.3).sub.2 --.
Aromatic polyamides represented by the general formula (I) are generally excellent in mechanical strength, heat resistance and dielectric characteristics and have uses as synthetic resins. Besides, by appropriate heating these aromatic polyamides undergo a dehydrating and cyclizing reaction and turn into corresponding polybenzoxazoles, which are promising as synthetic resins excellent in heat resistance, flame resistance, mechanical properties almost in every aspect and dielectric characteristics.
In general, aromatic polyamides are prepared by reacting an aromatic diamine compound with an aromatic dicarboxylic acid dihalide in an organic solvent. However, it is difficult to desirably control the degree of polymerization or molecular weight of the formed aromatic polyamides. Needless to mention, the properties of aromatic polyamides depend significantly on molecular weight. Aromatic polyamides not sufficiently high in molecular weight are insufficient in mechanical strength and heat resistance. Moreover, such aromatic polyamides are low in plasticity and in viscosity of their solutions and, therefore, can hardly be formed into films, fibers, etc. In respect of the aforementioned method of preparing aromatic polyamides, it is known that using equimolar quantities of the two reactants is effective for enhancing molecular weight of the formed polyamide, but in practice this theory applies only when the reactants are sufficiently high in reactivity. In the cases of preparing aromatic polyamides represented by the general formula (I), it has been difficult to obtain polyamides of sufficiently high molecular weight mainly because of low activities of aromatic diamines thus far used. Molecular weight of a polybenzoxazole derived from an aromatic polyamide is determined by the molecular weight of the polyamide, and the polybenzoxazole does not possess the aforementioned excellent properties when it is not sufficiently high in molecular weight.